Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high performance silver halide photography. Kofron et al disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having an ECD of at least 0.6 .mu.m and a thickness of less than 0.3 .mu.m exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. In the numerous emulsions demonstrated one or more of these numerical parameters often far exceeded the stated requirements. Kofron et al recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). Spectral sensitizations in all portions of the visible spectrum and at longer wavelengths were addressed as well as orthochromatic and panchromatic spectral sensitizations for black-and-white imaging applications. Kofron et al employed combinations of one or more spectral sensitizing dyes along with middle chalcogen (e.g., sulfur) and/or noble metal (e.g., gold) chemical sensitizations, although still other, conventional sensitizations, such as reduction sensitization were also disclosed. Subsequently the art has grouped with these outstanding tabular grains those having lower mean ECD's and lower average aspect ratios.
The overwhelming majority of camera speed tabular grain emulsions contain at least 70 mole percent bromide, based on total silver. The inclusion of at least low levels of iodide is known to improve further the speed-granularity relationships of tabular grain emulsions. Kofron et al, Wilgus et al U.S. Pat. No. 4,434,226 and Solberg et al U.S. Pat. No. 4,433,048 demonstrate speed-granularity relationships of silver iodobromide tabular grain emulsions.
Solberg et al as well as Piggin et al U.S. Pat. Nos. 5,061,609 and 5,061,616 disclose high bromide tabular grain emulsions in which higher levels of iodide are present in laminae having a thickness of less than 35 nm forming the major faces of the tabular grains. Solberg et al demonstrates that this structure can contribute to increasing photographic speed without increasing granularity. Piggin et al demonstrates that these laminae can reduce the susceptibility of tabular grain emulsions to varying photographic response as a function of the localized application of pressure (e.g., kinking or bending).
Johnson et al U.S. Pat. No. 5,164,292 discloses that speed losses attributable to incorporation of iridium dopants in tabular grain emulsions to reduce low intensity reciprocity failure can be offset by the incorporation of selenium as a dopant. When the tabular grains have higher iodide surface laminae, the dopants are compatible with retaining low levels of pressure sensitivity.
The utility of Group 8 (Fe, Ru, Os) metals to enhance speed is disclosed in Research Disclosure, Vol. 365, September 1994, Item 36544, Section I. Emulsion grains and their preparation, D. Grain modifying conditions and adjustments, sub-paragraphs (3) and (4) and Research Disclosure, Vol. 367, November 1994, Item 36736.